Television projection system



E. G. BAMBERG 2,298,808 TELEVISION PROJECTION SYSTEM Filed Ap'ril 2e,1941 /8 r -fa-.5. M/ROR n 7 Oct. 13, 1942.

4 Sheets-Sheet 2 Rlvy 1N Mamma/wiz; i PLANE Edivad G. Ramberg E. G.RAMBERG TELEVISIONJPRQJCTION SYSTEM "Oct 13, 1942.

4 Sheets-Sheet 5 Edward G. Bamberg rllvlll' Si os W if.. ...i

Oct. 13, 1942.

E. G. RAMBERG TELEVISIN PRQJECTION SYSTEM Filed Au 25; 1941 4Sheets-Sheet 4 I Il 1.9-

4 :inventor Edward @.Hamery PatentedOct. 13, i942 'miran stares fLTl-:NT

of Delaware Application Apriizs,1941,seria1N.390,4 .'z.ciaims...-fCl.17s-7. 5) My inye'ntion relates to systems for projecting televisionpictures or the like and particularly to projection systems in which theoptical system is of the type comprising a sphericalv mirror and aspherical-aberration correcting plate.

As explained in application Serial No. 248,569, led December 30, .1938,in the name of Daniel O. Landis, it has been found that alargetelevision y"arcanos l rELvIsroN rnomc'rrou SYSTEM Edward G.Ramberg,-Moorestown, N. ..,`asslgnor to Radio Corporation o! America, acorporation picture can be projected with suicient illumination and withgood denition by employing a specially designed optical system of theabovedescribed type. The present invention is an improvement on thesystem disclosed by the afore- When theliirst television projector madein accordance with the Landis invention was completed and tested byprojecting a picture from` only andwhen figured in accordance'with-tpresent invention, v

Figure 3 is' a-diagram which is referred inl'- describing the errorintroduced by the-thickness, Y

of thetube face, I i

Figure 4 is'adiagram which is referred to with o- Vreference to formulasemployed -in tracing a' -paraxial ray, A Y

Figures 5 and 6 are diagrams, not to scale, which are referred to inexplaining how a 'paraxial ray and a ray at a large angle to the aretraced through the sys;

kwhich arel traced through the system in order,

' to select the preferred values 'of tube face radius vand of focallength for the center of the correcting plate, and

Figures 8, 9 and 10 are curves which are calculated and utilized forselecting the above-mentioned preferred values of radius andfocal'length.

the end of a cathode ray tube, it was found that, c,

while the projected image was much brighter than that obtained by anypreviously employedoptical systems, theV detail -was less than had beenexpected for the known degree of accuracy of the system. v

I have discovered that the end of the cathode ray tube envelope, thatis, the tube iace, introduces an error in addition to the sphericalaber-l ration error of themirror which noticeably impairs the deilnitionof the projected-picture unless an additional correction is made. Inother words, while an image of good deiinition may be projected by fullycorrecting onlyfor the spherical aberration of the mirror, the qualityof the image -will be improved and will 'be of a very high order if theadditional correction for the tube face thickness is made.

In accordance with my invention, this error is corrected by shaping orguring the correcting plate of the optical system to correct not onlyfor spherical aberration of the mirror butnalso for the error introducedby the thicknessof'the end of the cathode ray tube.

The invention will be better understood f ro the following descriptiontaken in connection with the accompanying drawings in which Figure l isa side view of a television projection system embodying my invention,

Figure 2 is a sectional view of the correcting plate of the opticalsystem indicating the dierence, qualitatively, in the plate when iiguredto correct for spherical aberration of the mirror object rays from thescreen I3 must pass. The

cathode ray tube l0 may be of any suitable type and,'therefore, neednotbe described in`detail,;-

The optical system comprising the mirror vIl fand the correcting plateI2 is of the same type as that described and claimed in theabove-mentioned Landis application. The spherical mirror II by itselfwould project. an image having a, f

large amount of spherical aberration. Thaberration is removed by thecorrecting plate I2 which is properly shaped or iigured to give an imageof high quality on the projection screen.

The shape Aof the particular correcting plate ilto the plane of theprojection screen. As stated in the above-identified Landis application,the use of the spherical mirror and the correcting plate for correctingspherical aberration is based upon the principle of the well knownSchmidt hef the tube envelope; This end of the ltube has a 'certainthickness of glass through which the i2," as gured in accordance'with myinvention,

is represented in solid lines, while the outline of the correcting plateas would be figured without correcting'for the tube face thickness isindicated in dotted lines.

The fact that a substantial part of the total error in a system of thteabove-described type is introduced by the end of the cathode ray tubewas discovered fromv the following considerations. For the particularsystem described in this application (which isonehaving a 30-inchmirror) it is assumed that the maximum permissible ratio A ofthediameter of the consequent circle of diffusion in the image to thediameter of the image is 0.002. Itis assumed that the axial position ofthe tube'is adjusted for best focus of the paraxial rays and'that,th'excorrecting plate has been iigured to make the spherical aberrationzero for zero thickness of the tube face. "Since itis assumed that anobject without van intervening thick tube race is projected with perfectdenni- 1- tion, the value A also represents the maximum permissibleratio of the diameter of the consequent circle of diffusion in thevirtual image of the object produced by the tube face considered as' alens to the diameter of the object.

Fig. 3 shows a light ray`traced from a point of the object which is onthe axis, through the sin a=n sinv a-' y=t tan a Let p be the radius ofthe circle of diffusion due to spherical aberration on virtual image ofthe 3b-lect having a diameter d. Then a p=AI tan a A or A= 2 t(tan a sina d "n' V ,1f-T- h2 a Solving for t for the case where d=7 inches c:aperture angle=4245' t :0.053 inch 2At tanga "ff:-

Since the projection tube of the size assumed should have an envelopeabout three times this thick at the large end or face to withstand theatmospheric pressure, it is now apparent that the circle of diffusion inthe image will be too :large unless an additional correction isprovidedlin the optical system.

v One specific optical system ,designed in accordance with my inventionwill now be 'described by way of example. The method of designing thissystem will also be given, and, since the method applies to a system ofany size, it will be clear to those skilled in the art how to apply theinvention to optical systems having different lsize mirrors, differentprojection throws, etc.

f In this particular system, which is illustrated in Figure l, a frontsurface mirror inches in diameter is employed. A correcting platediameter of three-fourths the mirror diameter was selected, since thisresults in even light distribution over the projection screen for thecase where the central portion of the mirror is masked to\\ improvecontrast as described and claimed in jection system.

length of the mirror are determined by deciding what maximum apertureangle a is permis-- sible from. the point of view of aberration in thesystem'. The angle which was adopted is 4245', corresponding to anf-number of 0.737. The corresponding radius of curvature of the mirroris! R=32.4 inches.

The size of the cathode ray tube iluorescent screen is such as to give,at least approximately, Vmaximum .brillianceof the projected image. Atube screen diameter d of 'I'inches was selected as being satisfactory.

The radiusof curvature r of the tube screen should be made approximatelyequal to, or slightf' ly greater than,onehali the radius of curvature ofthe mirror in order that the image will be sharp over the entire surfaceof a dat projection screen. A'curvature r of 16.7 inches was selected bya procedure which will be explained hereinafter. 'I'he type of glass forthe tube face is Pyrex, with al mean index of refraction equal to 1.474.The thickness of the end of the tube or tube face is 1% inch.

The object and throw distances S and S', respectively, may now bedetermined by the approximate formulas:

where R is radius of mirror, m is the magnification, and f is the focallength of the correcting plate at itsI center or apex. In theseformulas,

such 'parts in the path of a ray that are glass (i. e., the thickness ofthe cathode ray tube face and the correcting plate) are treated asequivalent to paths in air which are shorter in the in-vy plalned below,the value f=8.918, R=288.9 inches is selected.

November 29, 1940and entitled Reective pro- I The correcting plate iscrown glass with an index of refraction equal to 1.532.

'I'he radius of curvature and, hence, the focal the optical system,. thevalue of S thus obtained would be an approximate vvalue and would beused as a check on other calculations. As explained hereinafter, theexact value of S is obtained by means of a more Aexact magnificationformula..

With respect to the selection of the focal length ,f of the correctingplate, while an image which is perfectly sharp at the center can beobtained by the proper shaping of the correcting plate for any value ofthe central refracting power or focal length f ofthe correcting plate,the quality of the image off the axis isdependent on the choice of Here'I'he ray is next refracted at the curvedV surface of the correctingplate indicated by subscript 3. The' radius of curvature of the cor-V"recting plate-at its center may now be found by the formula the valueof f. In order to obtain the optimal value of f, numerous off-axis'raysshould be calculated as will be explained hereinafter. Thus, by trialand error, the best value for f can be determined. This value of f isnormally used in the where M is the magnification,

, (J'=8.918R), which corresponds to the object dis# tance thusdetermined and to the given magnifi-4 cation, is obtained by tracing aparaxial ray from Solving for r,

curvature and the focal length is:

of the correcting plateat its apex (and also the exact throw distanceS). The value f will be Athe exact `focal length which will bring therays yto focus at the desired throw distance. This ray tracing is done,as explained below, by means of the following Well known formulas forparaxial rays, in which s is the object distance and s' is n the imagedistance as illustrated in Fig. 4:

Reflection at a mirror of'radius R: V 1/si-l/s-i` 2/R Referring to Fig.5.V (not to scale), a paraxial ray will now be traced by the formulas Ithrough the system beginning with the center of the object at o. Inorder to illustrate the ray path, the ray in this figure is shown at alarger angle to the axis than a paraxial ray. Firsttracing the ray`through the glass face of the tube and into air, from the formulan/s'=n/s+(n''n)/r, we can iind si', r in this instance being thecurvature of the outside surface of the tube face.

The ray is next reflected at the Vmirror.` For this reflection theobject distance (si) equals the distance from the tube face to themirrorplus s.

From the formula 1/s1=1/si+2/R, the subscript 1 referring to the mirrorsurface, we find that s'1= 873.9 (is negative since a paraxial raydiverges slightly after leaving the mirror).

The ray is next refracted at the plane surface of the correcting plate,which will be indicated by the subscript 2.

For lthis refraction, the object distance (sz) equals sfr plus `thedistance from the mirror face ni particuiar, an the pcint h=o, wam- 0,'as is usedin the process of obtaining the coefficients in the equationfor the correcting plate curve.

The equation for the correcting plate curve is:

tu isV the thickness of the correcting plate at the axis and t is thethickness of the plate at the radial distance h from the axis. For theapertures here considered, lit is satisfactory to break oi the seriesbeyond the term with ha. 'I'he slope of the corresponding plate curve isthen given by v dt/dh=2ah+4bh3+6ch5+8dh The coeicients may be determinedas fo11ows:' The first coeiiicient, 2a, is the reciprocal of the centralradius of curvature of the correcting plate, or 2a=1/r.v This can beshown as follows: The reciprocal radius of curvature r of any curve,such l t, regarded as a function of the independent variable h [that is,of the curve t=t (h)-lis, according to a well known lawof analyticgeometry, givenby L dit shown by substituting h=0 in the above equation'forl dt/dh. l Hence, at this point, that is, at the vertex of thecorrecting plate curve,

A differentiation of the equation for dt/dh with respect to h yields forh=0 en, dhza Therefore As to is fixed to begin with, only three furthercoeiiicients, b', c and d, need be determined to fully establish theshape of the correcting plate.

The procedure for finding the coefficients b, c and' d is as follows:From five to seven rays leaving the center point of the object at anumber of more or less evenly spaced angles to the axis are calculatedthrough the system by the aid of the well known trigonometric formulasgiven below with reference to Fig. 6 (not to scale). These formulas arefor rays in a meridional plane, that is, in a plane containing the sin8= (n/n) sin 0 II s=stan/tan9 Refiecton at mirror:

`sin i= sin r sin i In the above Formulas II, the object and imagedistances are represented by s and s', respectively. The angle 0 is theangle of incidence of the ray with respect to the axis, angle 6 is theangle the refracted or reflected ray makes with respect to the axis, iis the angle of incidence of the ray at the surface under consideration,and the angle 'i' the vertex of the correcting plate by means of theformulas for refraction at a plane surface.

Having traced a number of rays by the Formulas II fromI the axial pointof the object through the system up to the plane tangent to the vertexof the correcting plate curve, the height ofincidence h and angle ofincidence 0, with respect to the axis, are obtained for each. It is thenpossible to find that slope of the plate surface which will just bendthe incident ray so that it will. strike the axis at its intersectionwith the image plane. This slope is found as follows:

The throw distance S being known the anglelo of the refracted ray withthe axis is given byv n application of Snells law to the case in quesisthe angle of refraction or reflection at the surface in question.

Each of the several rays is traced throughgtheM system from the centralpoint o of the object in steps similar to those described for a paraxialray.

First considering the refraction caused by the tube face. the imagedistance s' and the angle 0' that the refracted ray makes with the axisare found by the formulas for refraction at a surface of radius r. Thepoint at which the ray strikes the mirror is now found.

By the formulas for reflection at the mirror, the point at which the raystrikes the plane surface of the correcting plate is found.

The ray is next traced to the plane tangent tion then yields for theangle of incidence a the curved'surface of the plate:

sin (0"-0) tafn t1v-cos (0-0) I and the slope of the curve is given,finally. by

Table I shows the results for h and dt/dh for the rays calculated forthe system being described.

Table I Anale of emer- Initial angle dI/dh Y (in gfazs) gentcb'om h(from tan van' 637' 1. 900585 0120150 8 l 1148 3. 371326 0199484 1311)"17l 5. 4491K@ .0267316 Three of the slopes dt/dh thus calculatedfrom tan pand the corresponding values of h are then substituted inturn'in the equation Thus the values of h and dt/dh substituted in theequation are those for the rays leaving the center point of the objectand making 13, 22 and 2730'to the axis. Solving the resulting threesimultaneous equations by determinants, the following values were found:4b=5.08994.10-5; 6c=8.72839.108;

The depths of the curve as a function of h for this system is shown inTable II below:

The remaining pairs of values of Jt/dh and h are used to check t1 acorrectness of the calculation and to determine the accuracy with which#usavo the curve represented by thepower series lits vthe ideal curve.The maximum deviation in slope which'has been found in any one case forthe 7 2 2. 5" plate.is34 seconds of arc, corresponding to \circle ofdiffusion with a diameter equal to 0.0004Nof that ofthe picture.

If aconstant thickness to has been used in .tracing the rays through thesystem, a certain error will result; abetter approximation can be=obtained by deriving the thickness at the points of incidence of theseveral rays from the formula s with the values for b,.c and d justdetermined the iirst stagewas found to have a diameter of less 0.0003ofA that of the picture. For plates with. much'steeper curves there mayvbe an advantage 'in going to' 'the second approximation, but even herethis can be avoidedif the thickness variation'is estimated from theproiiles of plates which have already been calculated.

lThe shape of 'the 22.5" correcting plate nally adopted is'foud to begiven by It has sai-minimum' thickness of 0.57207 inch an" h=l0.0639i'nch and curls up' slightly at the edge, Y'having there'(at h=ll.25inch) a thickness of 0.58598 inch. .Y It has previously been stated thatthe best radius of curvature r for the tube face has been selectedtomake the image fall in focus at all points on a flat projection screen`and that the focal length] of the correcting plate at its center isselected togive good marginal denition of the projected image as well asgood definition at its center. The process by which these values r and fare selected will now be described more fully.

The procedure, when calculations are tlrst begun on 4a new system, is.to select values for r and f that look like reasonable values. The sys--tem will then be calculated'using these values and the correcting platecurve is obtained. (This procedure has been described in detail, the de`scription being, of course, for a nal calculation with the nallyselected values of r and f.)

The next 'step is to Vtrace-a plurality of off-axis rays through thesystem to iind outl howmuch they deviate horizontally and verticallyfrom a principal ray leaving the same'oi-axs point on the object. Theprincipal ray is represented by a straight line drawn from the saidofi-axis point through the centerof curvature of the spherical mirror.

Seven oi-axis rays ae traced inv the particular example being given, veof them being in a vertical meridional plane (a meridional Plane beingone in which lies the optical axis of the system), and two of them(which are referred to as skew rays) being at an angle witnrespect tothe meridional plane and in a horizontal plane passing through theoli-axis point. The types of o-axis rays just referred to areillustrated in Fig. 'I where a plane surface is represented at P to aidin lillustrating the above-mentioned vertical and horizontal planes. .IBy means of this ori-axis ray tracingf which L? IIormallyyfthis'l is notnecessary: for' the 22.51

is done by means of the trigonometric formulas previously given and bymeans of the sk ew ray formulas given hereinafter.: there is found thehorizontal and vertical distances separating the 5 point on theprojection screen at which the offaxis ray falls and the point at whichthe principal rav'falls. After these deviation distances have been foundfor a system including the iirst calculated 1d correcting plate (havingthe rst selected value of f) and including .a tube with the tube facehaving the ilrst selected value of r, a. diierent value for r isselected and theseven ofi-axis rays are again traced through the systemusing the may now be plotted by drawing a straight line through eachpair of deviation points obtained with the two values of r. It will benoted that an additional pair of skew ray curves are plotted for skewrays on the opposite side of the vertical plane and symmetrical withrespect to the two skew rays traced through the system. From thesecurves, the value of the tube face radius'r which gives the minimumspread for a given cor- Yrecting plate of focal length f may bedetermined. As indicated by the vertical. arrows in' Fig. 8, thisminimum spread for .the example plotted is obtained if the tube faceradius 1' has a value of about 16.7 inches. It will be noted that thispoint o minimum spread is not necessarily thepoint either of minimumhorizontal deviation or of minimum vertical deviation, but, sinstead,-is a point where the deviation is the least, considering thetwo deviationstogether. The procedure for tracing the lve oi-axis raysin a meridional plane up to the plane surface ofA the correcting plateis the same as previously described for rays leaving the center of theob-` ject` at a 'substantial angle,`the same trigono- 1w'metric formulasbeing used. The ray tracing from the curved surface to the projectionscreen is done"as follows:

Referring to Fig. 6 andto the angles indicated thereon for a. rayincident to the curved surface of the plate and refracted thereby, theray strikes the curved surface at a distanceh from the axis which isfound by the equationh=h2 +t2 tan 0 where 'ha is the distance from the'axis to the point at which the ray strikes the ilat surface of theplate. and where ta is the thickness of the plate at this point and isalso almost exactly the thickness of the plate at the -very slightlydifferent and yet unknown distance h. J

The value of h thus found-is substituted in the equation ydt/dh=2ah+4bh+6ch5+sah1 to obtain' the velue 'of auchian e Then theangle 0 maybe calculated since imp-9 sin i=1iz sin i where -m is theindexof refraction of plate, and.

Y '=i' I I Finally the height of incidence H on the projection screen is-ltr- 41+[zr-Hinwil)1 ten o" AZISH-Ho Y where H is the height ofincidence -of the prmcipal ray on the projecti\on screen. In tracing thetwo skew rays it is convenient sam'e f correcting plate. The 'curves ofFig. 8 'V a and car/e determined as follows:

, ttefraction at a spherical surface of radius r:

of f to obtain vsuflcient points to plot a curve of minimum spread vs.l/f, From this curve, there is selected the focal length f that givesthe least spread, this being the value that is used in the finalcalculation of the sys-,

tem.

.There is also plotted a curve of optimum tube face radius 1' vs. l/f asshown in Fig. 10 from which is selected the best value of 1' for thevalue of f just chosen from the curve of Fig. 9.

1' It will be seen that, by the above-described consideration ofofi-axis "rays, vthe optimurr' values have been chosen for the tube faceradius r and for the focal length f of the center of'the correctingplate, whereby the system will project aberration, this type ofaberration being small as compared with the geometrical aberrationscon.-

an image of excellent quality at the margin as well as at the center ofthe image.

It may be noted that the .system which has been described is wellcorrected vfor chromatic.

sidered in the foregoing calculations. With respect to the shape orcurve of the cor- Refraction at alplane:

a, a'im'mr-l) 1+a2+c2 +111f2 where It will be apparent that, by means ofthe foregoing equations, a skew ray may b e traced from point to pointthrough the system to determine the point at which it falls on `theprojection screen.- It is then a simple matter to calculate the distancehorizontally and the distance vertically that this point is spaced froma corresponding principal ray.

recting-plate when it is figured in accordance" with my invention ascompared with its shape when figured without taking into account thethickness of the tube face, it has been pointed out with reference toFig. 2 that thecorrecting plate in the improved system is relativelyless concave at the edge. This diierence in the corrected anduncorrected curve of the correcting plate may be expressedmathematically as ex'- plained below. The correcting plate curve with-40 out tube face correction may be defined as fol-v where the distances' and the angles i9l and 0' are those shown in Fig. 6 atthe tube faceand where n is the index of refraction of the tube face.

Let At=differencein thickness of a correcting plate which is notcorrected for the' thickness of the tube face and of a correcting platewhich .is so corrected. The ray leaving the center point of the objectat the angle 0 will, continued backward after Having traced the oli-axisrays` through the system for onecorrection plate of focal length f andfor two values of tube face radii 1' whereby a set of curves such asthose in Fig. 8 is obtained, the next step is to select another value offocal length f for another correcting plate and repeat the procedure;again tracing through a number of olf-axis rays. Thus, there is obtainedanother group of curves similar to those in Fig. 8, but for a differentf value.

For this second f value, the tube face radius 1 that will give minimumspread is again selected. This procedure is repeated for several valuesrefraction at the tube face, intersect the virtualA i image plane atadistance om the,axis. It, as with the correcting plate uncorrected forthe thickness of the tube face,

the virtual object plane is imaged sharply on the projection screen, theactual ray leaving the object at the angle 0 will strike the projectionscreen a distance 5 from the axis, m being the magnification. Thus, i

as shown in Fig. 9.

geteste l to correct the plate for the tube face thiol-ness its slope atthe height of incidence o1 the ray considered must be changed by anamount d: AIE v such that the angle oi refraction at the correcmg plateis increased by an amount i. e. jest enough to cause the ray considerecto As, in order toproduce an angular deecaflon As' the slope ci thereiracting surface has te be changed by an angie 1 dt dat such that A6'A A=nz-1 the needed change in the slope of the correctirg plate is whereS' is the throw distance and nz is the irex of refraction of thecorrecting plate` As A di -2s Yf 1 cos 6' Where R is the radius ofcurvature ci the spherical mirror. Again, to a ifirst approximationwhere F is the focal length of the system We have now found 'now thecoecient la, c and d in the equation for the correcting peste Thenumerical values for Ab.. Ac and Ad are given below for the particularprojection unit described in this application,

ca -275.0543 A6- "Lirleosztsaof-m lo 7 L 18m10.516s ad meaeszsanf-7840-v Correcte Uncorrected "c: 1.2725-10-5 {1.293540-5) e iessrsw(Lessors-2) d: 5,5916-:101i (Meerlo-11) I claim as my invention:

l. A projection system comprising a concave spherical mirror, acorrecting plate positioned at least appro. ately at the center or'curvature of mirror. and a projection tube having an objc-:t surfacetherein which is positioned at a coniugate focus of the system andhaving a trans- 5 parent refractive end or tube face of a certain 1 24eQ tthickness, said correcting plate being gured to correct both for thespherical aberration of the mirror and for the error introduced by saidthickness of trie tube face.

2, A projection system comprising a concave spherical mirror, acorrecting plate positioned at least approximately at the center ofcurvature of said mirror, md a cathode :ay tn'ee having a luminescentscreen therein which' is positioned at a conjugate focus of the systemand having a transparent refractive end or tube face of a certainCxickness, said correcting plate being gured to correct both for thespherical aberration of the mirror and for the spherical aberrationintroduced by said thickness of the tube face.

3. A projection system comprising a concave 1 sos K 1 (111-, h5 1-p f l01 -nrW/h spherical mirror. a correcting plate positioned at leastappremmateiy at the center o crr'v'atuse of coniugate focus of thesystem and hnving s transv parent refractive end or tube fece ci certainthickness, said correcting plate being gured to correct for thespnericai aberration o! the mirror and for the edditiona errorintroduced by the refraction caused by the end of said tube whereby saidplate relatively leas concave on the edge than a correcting platefigured tc correct only for the spherical aberration of the mirror.

4. A projection system comprising a concave spherical mirror, acorrecting plate positioned et least approximately at the center ofcurvature of said mirror, and a projection tube having an object surfacetherein which is positioned et e. coniugate focus of the system andhaving a t transparent refractive end or tube ieee ci a certainthickness, said correcting plete being figured to correct both for thespherical aberration o the mirror and -or the error introduced by saidthickness of the tube fece, the correcting plate curve beingsubstantiaily defined by the equation escasos l. end or tuoe face ofsubstantiel thickness through which the light rays from seid 'iightimage pass, and encpiical system including a substantisily sphericalmirror having its concave surface area positioned to receive said lightrays from the iml ege, and an aspherical zone plete positioned ex termalto the path of the light projected from the light image to the reflectorand located at or near the center of curvature of said mirror to receivethe reected light from the reflector, said zone plate having suchcurvature as to correct both for the spherical aberration introduced bythe rellector and for the spherical aberration introduced 'oy saidthlclrne of the tube face,

where-t is the thickness of the correcting plate et distance h, h ic the:radial distance meee ured from the center of the correcting piste, tuis 35 Less of the toire fece, n is the index of refraction 5,5

of the tube face, m is the index ci refraction of the correcting plate,and R is the radius of curveiture of the spherical mirror.

5. im image projection device comprising e. cathode ray tube having abidimensional imsge eren on which there may be produced a light image,said tube having a. transparent refractive end or tube face oisubstantiel thickness through which the light rays from said light imagepass, and aiu optical system including o concave reectng surface ofrevolution having its conve surface avea. positioned to receive seidlight rays from the image, and an aspnerical zone plate positionedexternal to the path of the light projected from the iight image to thereiiector and positioned to receive the reected light from the reector,seid zone plate having such curvature as to correct both for thespherical aberration introduced by the reflector and for the sphericeiaberration introduced by seid thickness r whereby the optical systemincluding the reflector and tire sone plete is adapted to form aprojected spherical mirror having its concave surface aree.

positioned to receive said light rays from the imese, and an aspirericeizone plate positioned extel-nal to the path of the light projected fromthe ghi image to the reector and located at or near the center ofcurvature of said mirror to receive the reflected light from thereflector, said zone,

curvature also being such as to reduce the seid spheril aber-intiem to aminimum in n vievv'ir-ty area located at e, finite distance from thesaid zone plate.

EDWARD G. RAEx/IEE'G

